Subject: Re: [PATCH] rethook: Reject getting a rethook if RCU is not watching

From: Jiri Olsa <[email-masked]>
Date: Thu, 26 May 2022 16:49:26 +0200

On Thu, May 26, 2022 at 11:25:30PM +0900, Masami Hiramatsu wrote:
> On Tue, 24 May 2022 19:23:01 -0400
> Steven Rostedt <[email-masked]> wrote:
> 
> > On Sat,  7 May 2022 13:46:52 +0900
> > Masami Hiramatsu <[email-masked]> wrote:
> > 
> > Is this expected to go through the BPF tree?
> > 
> 
> Yes, since rethook (fprobe) is currently used only from eBPF.
> Jiri, can you check this is good for your test case?

sure I'll test it.. can't see the original email,
perhaps I wasn't cc-ed.. but I'll find it

is this also related to tracing 'idle' functions,
as discussed in here?
  https://lore.kernel.org/bpf/[email-masked]/

because that's the one I can reproduce.. but I can
certainly try that with your change as well

jirka

> 
> Thank you,
> 
> 
> > -- Steve
> > 
> > 
> > > Since the rethook_recycle() will involve the call_rcu() for reclaiming
> > > the rethook_instance, the rethook must be set up at the RCU available
> > > context (non idle). This rethook_recycle() in the rethook trampoline
> > > handler is inevitable, thus the RCU available check must be done before
> > > setting the rethook trampoline.
> > > 
> > > This adds a rcu_is_watching() check in the rethook_try_get() so that
> > > it will return NULL if it is called when !rcu_is_watching().
> > > 
> > > Fixes: 54ecbe6f1ed5 ("rethook: Add a generic return hook")
> > > Signed-off-by: Masami Hiramatsu <[email-masked]>
> > > ---
> > >  kernel/trace/rethook.c |    9 +++++++++
> > >  1 file changed, 9 insertions(+)
> > > 
> > > diff --git a/kernel/trace/rethook.c b/kernel/trace/rethook.c
> > > index b56833700d23..c69d82273ce7 100644
> > > --- a/kernel/trace/rethook.c
> > > +++ b/kernel/trace/rethook.c
> > > @@ -154,6 +154,15 @@ struct rethook_node *rethook_try_get(struct rethook *rh)
> > >  	if (unlikely(!handler))
> > >  		return NULL;
> > >  
> > > +	/*
> > > +	 * This expects the caller will set up a rethook on a function entry.
> > > +	 * When the function returns, the rethook will eventually be reclaimed
> > > +	 * or released in the rethook_recycle() with call_rcu().
> > > +	 * This means the caller must be run in the RCU-availabe context.
> > > +	 */
> > > +	if (unlikely(!rcu_is_watching()))
> > > +		return NULL;
> > > +
> > >  	fn = freelist_try_get(&rh->pool);
> > >  	if (!fn)
> > >  		return NULL;
> > 
> 
> 
> -- 
> Masami Hiramatsu (Google) <[email-masked]>